One common theme could be conflict. Since Eric is hurting Tris, there would be the obvious physical or emotional conflict between them. This can create a lot of tension in the story.
I'm not sure about specific 'eric hurts tris fanfiction' works off the top of my head. But you can try searching on fanfiction websites like Archive of Our Own. There are usually a lot of fan - created stories in different genres and themes related to various characters.
One common theme is the exploration of their relationship dynamics. Since they have a complex relationship in the original work, fanfictions often dig deeper into how they interact, whether it's as friends, rivals or something more.
Eric might also use his position of authority. If he's in a position of power within the story's setting, he could simply order Peter to stop. For example, if Peter is violating some rules of the group or faction they belong to, Eric could threaten him with punishment if he doesn't let Tris go.
One common theme could be struggle. Given the name 'Four Hurts Tris', it might involve the challenges and hardships that Tris faces, perhaps related to her relationship with Four or other aspects of her life in the story's universe.
Another theme could be the idea of forbidden love. If there are certain circumstances in the original story that make their relationship difficult or against the rules, this can be a popular theme in the fanfiction. Writers can then explore how they deal with this forbidden aspect, like sneaking around or fighting against the system for their love.
Well, I know that some fanfiction platforms have a tagging system. Check those tags for 'tris and eric' and explore the ones that seem interesting. Sometimes, you might find hidden gems that way.
A theme that can't be ignored is the transformation. Sometimes Tris undergoes a transformation, either physically or in terms of her understanding of the world. It could be that she becomes more like a vampire or gains a new perspective on life. And the overarching theme of good vs. evil, with Eric and Tris often on the side of good, fighting against the more malevolent vampires or entities in the fanfiction.
One common theme could be the challenges they face as a couple during the pregnancy. It might include things like dealing with their different personalities while preparing for a new life together.